Apparatus for grinding facets on gem stones, diamond styli and other relatively hard materials typically include a disc shaped lap having an abrasive material disposed thereon. The disc is rotated by a motor and the part to be lapped, which is held in proper position by a holder, is urged into engagement with the spinning surface of the lap. The holder is made to oscillate or undergo other movement thereby causing the part to traverse a given portion of the lapping surface.
In one type of lapping machine the lapping surface is used only once. As the disc shaped lap turns, the part that is being lapped is advanced along a radial which emanates from the axis of rotation. The magnitude of the advancement is coordinated with the rotational speed of the lap so that the part always encounters an undisturbed portion of the lapping surface. It is customary, in this type of lapping machine to rotate the disc shaped lap at a constant angular velocity, approximately 250 RPM for a lap of about 14 inches in diameter. This speed was selected to achieve an acceptable over all cutting efficiency which is maximized at one point on the lapping surface. This, of course compromises cutting efficiency at other points on the lapping surface.
Such a compromise has little consequence in most lapping applications such as gem stone lapping. However, when lapping high precision parts such as diamond styli for video disc pickup applications the compromise is significant. Such diamond styli typically have a tip that is only about 2 micrometers wide and 4 micrometers long including several facets. The size and relative positioning of the facets are critical to proper performance of the finished stylus. Further, variable cutting efficiency, as the stylus progresses across the face of the lap, tends to frustrate the goal of repeatably producing finished styli within tolerance and of high quality utilizing automated equipment.